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Re: [TowerTalk] [Bulk] Re: 90 degree coax bend atop a crankup tower

To: ve4xt@mymts.net
Subject: Re: [TowerTalk] [Bulk] Re: 90 degree coax bend atop a crankup tower
From: Grant Saviers <grants2@pacbell.net>
Date: Fri, 05 Jun 2015 07:39:02 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Tis the simple answer. Use a Kellems grip and then there is no stress on the bend. They are available to slide over the end of the cable or as "lace on" so they can go on an established length or where a connector on the cable is larger than the expanded diameter. LMR400, Buryflex, LMR600UF and the like have a specified tensile strength of 350 to 400# and a weight per foot of 0.07 to .16#/ft so one grip at the top handles up to 110' crank up with better than a 2:1 safety factor.

Grant KZ1W

On 6/5/2015 4:32 AM, ve4xt@mymts.net wrote:
Wouldn't holding the coax with a sling-type grip in such a way that the bend 
itself is a tension-free loop, solve both problems?

The issue in both cases is the unsupported weight of the coax pulling on the 
bend, yes? So if you took all the weight off the bend...

Or am I missing something?

73, Kelly
ve4xt


Sent from my iPad

On Jun 5, 2015, at 12:15 AM, "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net> 
wrote:

Coax has two bend radius listed. A repeatable radius and a one time bend.  It's a 
good idea to stay well away from both of these.  The one time bend can be a very 
short radius. Short enough that the center conductor will want to migrate off center 
given enough time just laying on a flat surface.  The repeatable bend radius is 
usually quite a bit larger and "should not damage the coax over a given number 
of bends. No, I don't know the number, but the minimum repeatable bend radius can 
still stress the coax.

Now hang a load on the coax in a vertical run.  The center conductor will tend 
to migrate downward.  In a 100 ft vertical run it's often enough for N type 
connectors to lose continuity.   If the coax is supported only from the top 
with a relatively sharp bend, the center conductor will migrate off center.  
That's why several small loops evenly spaced up the side of the tower will 
remove the load from that top support.

There are several methods of supporting the coax from the top on crank up 
towers.   Wire mesh and rope slings do a good job of holding the coax, but do 
nothing for the linear migration of the center conductor.  Several commercial 
ones linked to on here appear to do a pretty good job,

Too sharp a bend causes the off center migration, while too wide a bend will allow the 
linear migration.  There has to be "a best radius" for each size and type of 
coax.  I'd choose about twice the repeatable bend radius.  No concrete proof, it just 
looks right. Not exactly a scientific approach.

Tall, crank up towers put a lot of stress on the coax supported only from the 
top.  Unfortunately supporting the coax at intermediate heights brings a whole 
new can of worms.

73

Roger  (K8RI)

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